Question: On the first 3 tests of her math class, Vanessa got an average score of 88. What does she need on the next test to have an overall average of 91?
Let her score on the next test be $x$ The sum of all of her scores is then $3 \cdot 88 + x$ The same sum must also be equal to $4 \cdot 91$ Solve: $x = 4 \cdot 91 - 3 \cdot 88 = 100$.